The 'in love' phase lasts about a year.

The honeymoon phase doesn't go on forever.

According to a 2005 study by the University of Pavia in Italy, it lasts about a year. After that, levels of a chemical called "nerve growth factor," which is associated with intense romantic feelings, start to fall.

Helen Fisher, a psychologist and relationship expert, told Business Insider that it's unclear when exactly the "in love" feeling starts to fade, but it does so "for good evolutionary reasons," she said, because "it's very metabolically expensive to spend an awful lot of time just focusing on just one person in that high-anxiety state."

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Two people can be compatible — or incompatible — on multiple levels.

Back in the 1950s and '60s, Canadian psychologist Eric Berne introduced a three-tiered model for understanding a person's identity. He found that each of us have three "ego states" operating at once:

• The parent: What you've been taught

• The child: What you have felt

• The adult: What you have learned

When you're in a relationship, you relate on each of those levels:

• The parent: Do you have similar values and beliefs about the world?

• The child: Do you have fun together? Can you be spontaneous? Do you think your partner's hot? Do you like to travel together?

• The adult: Does each person think the other is bright? Are you good at solving problems together?

While having symmetry across all three is ideal, people often get together to "balance each other." For instance, one may be nurturing and the other playful.

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The happiest marriages are between best friends.

Controlling for premarital happiness, the study concluded that marriage leads to increased well-being — and it does so much more for those who have a close friendship with their spouses. Friendship, the paper found, is a key mechanism that could help explain the causal relationship between marriage and life satisfaction.

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The closer a couple are in age, the less likely they are to get divorced.

A study of 3,000 Americans who had ever been married found that age discrepancies correlate with friction in marriages.

"A one-year discrepancy in a couple's ages, the study found, makes them 3 percent more likely to divorce (when compared to their same-aged counterparts); a 5-year difference, however, makes them 18 percent more likely to split up. And a 10-year difference makes them 39 percent more likely."

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If you get excited for your partner's good news, you'll have a better relationship.

In multiple studies, couples that actively celebrated good news (rather than actively or passively dismissed it) have had a higher rate of relationship well-being.

For example, say a wife comes home to her partner and shares an accomplishment. An "active-constructive" response would be the best, according to Amie Gordon, a social psychologist at the University of California at Berkeley:

• An active-constructive response from the partner would be enthusiastic support: "That's great, honey! I knew you could do it. You've been working so hard."

• A passive-constructive response would be understated support: a warm smile and a simple "that's good news."

• An active-destructive response would be a statement that demeaned the event: "Does this mean you are going to be gone working even longer hours now? Are you sure you can handle it?"

• Finally, a passive-destructive response would virtually ignore the good news: "Oh, really? Well, you won't believe what happened to me on the drive home today!"

"If you really are better at the dishes than remembering to call the in-laws, then that should be your job," she writes. "It'll take you less time than it'll take him, and it'll take him less time to have a quick chat with mom than it would take you, which means in the end, you've saved quite a bit of collective time."

Before 1850, couples got hitched for the sake of food, shelter, and protection. Then with the Industrial Revolution people had more leisure time, Finkel says, so we started looking for companionship in our partners. The '60s brought a yearning for personal fulfillment through relationships, which we continue to strive for today.

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You'll never get to know your partner perfectly.

After dating someone for a couple of years, you might feel like you know everything about them: what kind of toothpaste they use, which TV series they guiltily binge-watch, which foods nauseate them.

If you're moving in to 'test' the relationship, you're probably not so confident in it.

A 2009 study led by researchers at the University of Denver found that most couples moved in for other reasons besides test-driving their relationship before marriage.

But couples who did report testing the relationship were more likely to experience a number of negative emotions. For example, among testers, men scored higher on measures of depression and anxiety, and women scored higher on measures of abandonment anxiety. Both groups were less confident in the relationship.

"It seems to us that many people who think about testing their relationship by cohabiting already know, on some level, what the grade of that test may be; they are hoping that the answer looks better over time."

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If you're economically dependent on your spouse, you're more likely to cheat on them.

Contrary to popular belief, cheating isn't necessarily more common among high-earning couples. The link between income and infidelity is more nuanced than that.

Recent research from the University of Connecticut suggests that a person who is economically dependent on their spouse is more likely to be unfaithful — and that's especially true for a man who relies financially on a woman.

Interestingly, when women are the breadwinners, they're less likely to cheat. When men are the breadwinners, they're more likely to cheat.

We think everyone except our own partner is cheating.

You can't trust anyone — except your boo, right?

A 2015 University of Calgary study found that heterosexual undergrads think the average member of the opposite sex has about a 40% chance of cheating on their partner. But those same participants said their own partner had only a 5% chance of cheating.

The rate at which participants said they'd ever really cheated on their partner? 9%.

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Couples who appreciate each other are more likely to stay together.

In one University of North Carolina at Chapel Hill study, researchers had participants keep private daily diaries in which they recorded things their partner had done for them and how it had made them feel. As it turns out, couples who were more grateful toward each other felt that the relationship was stronger.

Meanwhile, another series of studies, led by a researcher at the University of California, Berkeley, found that more grateful couples were more likely to still be together nine months later.

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Eventually you realize that you're not one person.

Once you start living together, you may realize that you have different priorities and tolerances — like, for instance, what does or doesn't constitute a mess.

"People have to come to terms with the reality that 'we really are different people,'" says Ellyn Bader, a couples therapist. "'You are different from who I thought you were or wanted you to be. We have different ideas, different feelings, different interests.'"

It's a stressful — and necessary — evolution.

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When it comes to sex, quality is more important than quantity.

Researchers split a bunch of heterosexual, married couples into two groups: For 90 days, half continued with their normal sex schedule and half had sex twice as often. When the researchers measured how each group felt at the end of the experiment, the group that had doubled their sex frequency was in fact slightly less happy.

As the lead researcher behind the study told The New York Times, if you want to be happy, focus on quality over quantity.